Environ. Sci. Technol. 1999, 33, 3543-3551
Characterization of Solid Emissions from Atmospheric Fluidized-Bed Combustion of Two Czech Lignites J I Rˇ IÄ S M O L IÄ K , * , † J A R O S L A V S C H W A R Z , † V AÄ C L A V V E S E L YÄ , † I V A N A S YÄ K O R O V AÄ , ‡ JAN KUC ˇ ERA,§ AND V L A D I M IÄ R H A V R AÄ N E K § Institute of Chemical Process Fundamentals AS CR, Rozvojova´ 135, 165 02 Prague 6, Czech Republic, Institute of Rock Structure and Mechanics AS CR, V Holesˇovicˇka´ch 41, 182 09 Prague 8, Czech Republic, and Nuclear Physics Institute, 250 68 Rˇ ezˇ Prague, Czech Republic
We investigated composition and morphology of solid residues (fine and coarse fly ashes and bottom ash) from atmospheric fluidized-bed combustion (AFBC) of two Czech lignites that are currently used or that will likely be used for generating energy in Czech thermal power plants. The experiments were carried out on an experimentalscale 100 kW AFBC boiler using ash as a material of the bed. Coarse fly ashes were collected by a process cyclone; fine fly ashes were separated by a low-pressure Berner impactor. The elemental composition of fly ashes was examined by using INAA and PIXE and, for some samples, also by using ion chromatography. Morphology of ashes was investigated by SEM. The fine particles were found to be spherical or smooth aggregates; they were formed by alkaline-earth and alkali metal sulfates, substantially enriched by volatile trace elements. The coarse particles, both spherical and irregular in shape, were formed by partially transformed mineral inclusions remaining after burn out of organic matter. The results indicate that reducing the concentration of sulfur dioxide in the flue gas may reduce the emission of fine particles.
Introduction The negative impact of the power industry on the environment in the Czech Republic is a consequence of the structure of primary energy sources (coal 58.5%, oil 18.1%). The utilization of low-quality coal produces large quantities of fly ash containing toxic metals, especially As (1). The air quality field studies conducted in the northwest Czech Republic indicate that most of fine aerosol mass is produced by home heating and power plants, with combustion of lignites from the North Bohemian Basin in power plants that use both pulverized coal and fluidized-bed combustion units contributing about 30-80% of the average fine particle mass (2). As found in both laboratory and field studies on aerosol emissions from pulverized coal combustion (3-11) and fluidized-bed combustion (12, 13) fly ashes exhibit bimodal particle mass size distribution, indicating two distinct mech* Corresponding author telephone: (+420) 2 20390247; fax: (+420) 2 20920661; e-mail:
[email protected]. †Institute of Chemical Process Fundamentals AS CR. ‡ Institute of Rock Structure and Mechanics AS CR. § Nuclear Physics Institute. 10.1021/es9809399 CCC: $18.00 Published on Web 09/01/1999
1999 American Chemical Society
anisms of fly ash formation. The fine particle mode is formed by ash particles with mean sizes ranging from 0.01 to 0.1 µm that are generally thought to result from the vaporization of ash-forming constituents followed by their condensation. Although the submicron fraction usually contains less than a few percent of the total mass of the ash (8, 10, 14, 15), the vast majority of ash particles is in this range (5) and accounts for most of the surface area of the suspended particulate matter. The surface area-dependent heterogeneous condensation or surface reaction of vaporized elements then can cause elemental enrichment of the submicron fraction as observed in coal combustion investigations (16). The spherical and irregular coarse ash particles ranging in size from 1 to 100 µm are formed by mineral inclusions released after the burnout of organic matter of coal. The spherical particles are produced by fusion or partial melting of discrete mineral particles and coalescence of melted mineral inclusions. The nonspherical particles are formed by mineral inclusions that do not melt or that only partially melt due to high melting points. Recently, an intermediate mode has been observed in the submicron size range (17) that is proposed to result from partial melting of very fine mineral particles in the coal. As described below, the formation of fly ash from coal combustion is a complex physicochemical phenomena that depends on the type and composition of coal, the combustion facility, and the combustion conditions. Although in both pulverized and fluidized-bed systems the combustion of a coal particle includes drying, devolatilization, combustion of volatile matter, and combustion of residual char, the combustion conditions and hence behavior of individual elements during the combustion differ considerably. In pulverized coal combustion, the temperature of burning char particles is about 1800-2500 K (18-20). At these temperatures, silica (and also alumina) can react with carbon to produce a more volatile suboxide (21, 22). When the reduced vapor diffuses away from the burning particle, it can be reoxidized in a more oxygen-rich atmosphere to silica, which then can condense to submicron silica fume via nucleation and coagulation (19). Although formation of silica fume near the burning coal particle was predicted theoretically (19) and formation of silica fume (and alumina-rich fume) were observed during the heating of ash slag in a graphite crucible (22), silicon and aluminum are usually depleted in submicron aerosol from pulverized coal combustion (3, 6-8, 10, 15, 23-27). This might be caused by condensation of large quantities of Mg, Ca, Fe, and Na presumably in the form of sulfates, observed especially in the combustion of subbituminous coals and lignites (3, 6-8, 15, 25, 27, 28). In fluidized-bed combustion, particles of coal burn within the bed of vigorously moving smaller inert particles with bed temperatures between 1000 and 1300 K. Due to intense heat transfer from the burning particle to bed particles and percolating gas, the temperature of a particle is on average 200 K above that of the bed (29, 30). The high efficiency at low combustion temperatures makes the fluidized-bed technology very convenient for energy production. It can handle low-grade coals with higher sulfur content, the lower combustion temperature reduces the emissions of the thermal NOx, and easy application of additives can substantially reduce the emission of gaseous pollutants. The particulate emission and emission of trace elements are also expected to be reduced due to lower volatilization of elements at lower operating temperatures. Very limited data exist on the composition and morphology of particulate matter emitted from fluidized-bed comVOL. 33, NO. 20, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Coal Properties Centrum
Vrsˇ any
25.5 7.2 0.63 0.57 0.03 0.03 51.46 30.71
22.1 18.0 0.96 0.65 0.27 0.04 53.06 29.43
Ultimate in d.s. 73.05 5.57 0.96 0.58 19.74
70.96 5.45 1.00 0.80 21.75
Proximate moisture (%) ash in d.s. (%) total S in d.s. (%) organic S in d.s. (%) pyritic S in d.s. (%) sulfate S in d.s. (%) volatile matter in d.s. (%) heating value (MJ/kg) C (%) H (%) N (%) S (%) O (%) SiO2 TiO2 Al2O3 Fe2O3 Mn3O4 MgO CaO Na2O K2O P2O5 As Cd Cr Mn Ni Pb V Zn
Ash Composition (%) 45.14 2.03 29.73 6.52 0.048 3.20 6.88 1.06 0.72 0.29 Trace Element Concentrations (ppm) 13
